(core) Migrate import code from data engine to Node

Summary:
Finishing imports now occurs in Node instead of the
data engine, which makes it possible to import into
on-demand tables. Merging code was also refactored
and now uses a SQL query to diff source and destination
tables in order to determine what to update or add.

Also fixes a bug where incremental imports involving
Excel files with multiple sheets would fail due to the UI
not serializing merge options correctly.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3046

sandbox/grist/useractions.py

@@ -1550,8 +1550,9 @@ class UserActions(object):
     return self._import_actions.DoGenImporterView(source_table_id, dest_table_id, transform_rule)
 
   @useraction
-  def TransformAndFinishImport(self, hidden_table_id, dest_table_id, into_new_table,
-                               transform_rule, merge_options = None):
-    return self._import_actions.DoTransformAndFinishImport(hidden_table_id, dest_table_id,
-                                                           into_new_table, transform_rule,
-                                                           merge_options or {})
+  def MakeImportTransformColumns(self, source_table_id, transform_rule, gen_all):
+    return self._import_actions.MakeImportTransformColumns(source_table_id, transform_rule, gen_all)
+
+  @useraction
+  def FillTransformRuleColIds(self, transform_rule):
+    return self._import_actions.FillTransformRuleColIds(transform_rule)